What is the length of day at the equator? I recently told a friend with great certainty that at the equator the sun rose and set at exactly 12 hour intervals at roughly 6 a.m. and 6 p.m. (depending on the time zone) every day of the year. Now I'm not so sure.

You're right. It is always exactly 12 hours. There are a couple of different ways you can convince yourself that this is so, but I think the easiest one is this symmetry argument:

In the Northern Hemisphere, the length of the day is longer during the months when the North Pole is tilted towards the Sun and shorter during the months when it's tilted away from the Sun. The reverse is true for the Southern Hemisphere. The Equator is exactly halfway in between the poles. So it wouldn't make any sense for a day on the equator to be longer when one of the poles is tilted towards the Sun, and shorter when the other one is.

About the Author

Chris studies the large scale structure of the universe using the peculiar velocities of galaxies. He got his PhD from Cornell in 2005, and is now a Research Assistant Professor at the University of Western Australia.

Curious Minds Online

We have 3059 guests and one member online

How Many Were Here?

Total page views since 1997

101975295

Who are We?

Ask an Astronomer is run by volunteers in the Astronomy Department at Cornell University. Most of us are graduate students at Cornell, and all of us do this voluntarily, in our own time, fitting it in around our other work. Please take the time to browse our site and first try to use the resources online to find an answer to your question.